1. Field of the Invention
This invention relates to processes for the synthesis of three-dimensionally periodic structures and functional composites by the self-assembly of spheres, followed by one or more structure modification, infiltration, and extraction processes. These structures can be applied as thermoelectrics and thermionics, electrochromic display elements, low dielectric constant electronic substrate materials, electron emitters (particularly for displays), piezoelectric sensors and actuators, electrostrictive actuators, piezochromic rubbers, gas storage materials, chromatographic separation materials, catalyst support materials, photonic bandgap materials for optical circuitry, and opalescent colorants for the ultraviolet, visible, and infrared regions.
2. Description of Related Art
The art describes various means for fabricating articles with periodic structures that repeat on the scale of millimeters, such as by conventional machining methods. Methods are also available for the fabrication of articles having three-dimensional periodicities down to about 100 microns, such as desktop manufacturing methods. On the other extreme, three-dimensionally periodic structures with periodicities on the scale of angstroms can be obtained by conventional crystallization. In between these extremes there exists a manufacturability gap of from about 100 microns to about 10 nm, where it is presently difficult or impossible to fabricate three-dimensionally periodic structures from desired materials. The present invention enables the fabrication of such periodic structures in this manufacturability gap.
Two-dimensionally periodic structures can be created in this manufacturability bandgap using known methods, such as optical and electron beam lithography and mechanical embossing from lithographically produced surfaces. However, achievement of similar periodicity in the third dimension has provided the greatest problem. Limited success has been achieved in creating three-dimensionally periodic structures by the self-assembly of colloidal particles (especially colloidal particles that are spherical and nearly monodispersed in diameter). In addition, some researchers have been successful in filling porous periodic structures made of SiO.sub.2 spheres with other materials, including, metals superconductors, and semiconductors [see V. N. Bogomolov et al. in Phys. Solid State 37, No. 11, 1874 (1995) and in Phys. Solid State 39, No. 11, 341 (1997)]. However, methods have not been discovered for the elimination of the SiO.sub.2 spheres from the infiltrated structure, and the presence of these spheres can degrade the desired properties resulting from the infiltrated materials. Devising an overall process that preserves the structure of the three-dimensional array of infiltrated material, while at the same time enabling the extraction of the SiO.sub.2 spheres, represents a higher level of difficulty which has not been addressed by the prior art.
The lack of more success in prior research reflects several generic issues. In order to conduct high temperature infiltration processes, it is necessary to use a first matrix material (such as an array of crystallized SiO.sub.2 spheres) that is thermally and mechanically stable to above 300.degree. C. However, extraction processes have not been successfully demonstrated for such thermally stable matrix materials. One reason is that it is topologically impossible to extract such matrix materials (unless the preextraction processes of this invention are utilized)--because the spheres of the matrix material are buried in the infiltrated material. Even if this topological problem could be solved, the unsolved problem still remains of conducting such extraction of a high-thermal-stability matrix material (like SiO.sub.2) without disrupting the structure of the infiltrated material. There has, however, been some success in crystallizing low thermal stability polymers as matrix materials (which transform to a gas on heating), infiltrating these materials by low temperature processes, and then removing the original polymer particles by gas phase processes (resulting from polymer degradation). Specifically, Velev et al. [Nature 389, 447 (1997)] made three-dimensionally periodic shells of silica by using a chemical reaction to form the silica as a coating within polystyrene latex particle arrays, and then burning away the polystyrene (causing 20-35% shrinkage of the unit-cell parameter). Likewise, Wijnhoven and Vos (Science 281, 802 (1998)) made analogous crystals consisting of titania by assembling polystyrene latex spheres into a face-centered-cubic structure, chemically reacting tetrapropoxy-titane inside the polystyrene sphere structure (using up to eight penetration, reaction, and drying steps), and then burning away the polystyrene spheres (providing 33% shrinkage of the unit-cell parameter). A quite similar polystyrene-sphere-based method was used by B. T. Holland et al. [Science 281, 538 (1998)] to make titania, zirconia, and alumina. Such processes can avoid the above topological problem by using holes in the reacting coating layer (or layer permeability) to permit release of the gases produced by pyrolysis. However, this approach is generally unsatisfactory because of (1) inapplicability for materials that are most desirably infiltrated at high temperatures, (2) the difficulty of crystallizing the polymer spheres into well-ordered crystals having large dimensions, (3) the possible introduction of holes in the structure of the infiltrated material during gas evolution, (4) the occurrence of about 20-35% shrinkage of lattice parameter of the final structure relative to the initial structure, which can disrupt structural perfection, (5) inaccurate replication of the void space in the original structure (evident from the micrographs of the above references), (6) the lack of mechanical robustness of the polymer sphere assemblies (which again restricts the infiltration process), (7) the impossibility of obtaining complete filling of the void space of the original opal structure by the demonstrated chemical methods (so to obtain the volumetric inverse of the opal structure), and (8) the unsuitability of template removal by pyrolysis for the preparation of lattice structures comprised of thermally labile materials, such as polymers. As an alternative method, Imhof and Pine [Nature 389, 948 (1997)] have made periodic foams by using a sol-gel process to deposit materials in a self-assembly of monodispersed emulsion droplets. Barriers to application are provided by the lack of generality of this method, present inability to provide well-ordered materials of large dimensions by emulsion self-organization, the poor degree of order of the resulting product, and the large materials shrinkage during the drying step for the gel (about 50%).
What is needed and what the prior art has not provided is a means for forming three-dimensionally periodic structures with periodicities on the scale of 100 microns to 10 nm from arbitrarily chosen materials. Such materials are needed for a host of applications where the scale of the lattice periodicity profoundly effects properties. Moreover, the prior art has not demonstrated the ability to create the complicated, multicomponent structures needed for advanced device applications. The formation of these multicomponent structures requires the ability to conduct multiple infiltration and extraction steps without substantially degrading regularity, the ability to control structural channel dimensions independent of unit-cell dimensions, the ability to conduct infiltrations at high temperatures, and the ability to controllably engineer breaks in the continuity of infiltrated materials by melt phase processes--none of which have been demonstrated by prior art processes leading to either a opal replica structure or a more complicated structure. Also, methods are needed for the creation of three-dimensionally periodic nanoscale structures with less than 26% volume filling from thermally unstable materials, such as organic polymers (and especially elastomeric polymers and piezoelectric polymers), and such methods do not exist in the prior art. In addition, there are no available methods in the prior art for making a material that is a fully filled volumetric inverse of the void space of an opal structure, and materials with such structures are required for the applications described herein.